In developing a communications system, it is generally advantageous for a communications link to utilize the strongest signal feasible for improving signal quality and for providing sufficient coverage or range. With regard to signal quality, a stronger signal yields a higher signal-to-noise ratio (SNR). Also, a stronger signal propagates a further distance. However, the power level of transmitted signals must be constrained within limits. For example, in most situations, transmission power levels are regulated under rules imposed by governmental agencies such as the Federal Communications Commission (FCC). Indeed, this is important so as to prevent one or more powerful signals from interfering with the communications of other signals in the same frequency range. Other restrictions may be imposed by standards committees or may be self-imposed by a system in order to minimize interference where several signals are expected to exist simultaneously.
In view of the above, an important consideration in designing a communication system is its performance over a wide temperature range. It has been observed that the characteristics of a communication system change over temperature in such a way that its transmission power is affected. For example, while maintaining all other conditions constant, a communication system can transmit lower power levels at elevated temperatures and it can transmit higher power levels at very cold temperatures, and vice-versa.
Whatever the characteristics of a communications system may be, it is nonetheless desirable to closely monitor and control the transmission power. It is therefore important to know the transmission power levels of a communication system. A common scheme for monitoring or detecting a system's radio frequency (RF) power is through the use of a semiconductor Schottky barrier diode. RF detectors are essentially low sensitivity receivers which function on the basis of direct rectification of an RF signal through the use of a non-linear resistive element—a diode. Generally detectors using Schottky diodes can be classified into two distinct types: the small-signal type, also known as square-law detectors; and the large-signal type, also known as linear or peak detectors. In operation, a small-signal detector is dependent on the slope and curvature of the VI characteristic of the diode in the neighborhood of the bias point. The output of the detector is proportional to the power input to the diode. That is, the output voltage (or current) of the detector is proportional to the square of the input voltage (or current), hence the term “square law.” Large-signal detector operation is dependent on the slope of the VI characteristic in the linear portion, where the diode functions essentially as a switch. In large-signal detection, the diode conducts over a portion of the input cycle and the output current of the diode follows the peaks of the input signal waveform with a linear relationship between the output current and theinput voltage.
The square law dynamic range may be defined as the difference between the power input for a 1 dB deviation from the ideal square law response (compression point) and the power input corresponding to the tangential signal sensitivity (TSS). TSS is a measure of low level sensitivity with respect to noise. Normal operating conditions for the Schottky detector, a square-law detector, call for a large load resistance (100 kΩ) and a small bias current (20 μA). These normal conditions assure the minimum value of TSS, but not the maximum value of compression level.
One conventional manner of raising the compression level is by reducing the value of the load resistance, RL. But the sensitivity degrades by the factor RL/(RL+RV), where RV is the diode's specified video resistance. This degradation in TSS can exceed the improvement in compression, such that there is no improvement in square law dynamic range. Another conventional technique for raising the compression level is to increase the bias current. This also degrades the sensitivity, but the improvement in compression exceeds this degradation so square law dynamic range is increased. Although these approaches may improve a detector's general performance, they do not significantly improve a detector's dynamic range.
With regard to a communication system, it is important to know its transmission power level at particular temperatures of operation. Conventional approaches have used a power detector and a temperature sensor so as to develop a calibration table. In conventional calibration methods, the entire communication system is exercised at various temperatures while noting the output of the detector module. When placed in service, the communication system would then retrieve calibration data at a measured temperature so as to accurately measure the system's transmission power. Such conventional calibration methods, however, necessarily required that the entire system, or at least a large part of it, be placed in a temperature chamber. Because of the size and mass of such configurations, the calibration system is slow. Moreover, because an entire system is calibrated, any changes in components, such as upon a failure, required re-calibration.